This is extremely easy, as I am definitely not the person to spend time in the kitchen more than needed.

I used:
one can of tuna in water, with the water included,
half-cup of oatmeal,
a little bit of flax seed,
a sprinkle of garlic powder,
one egg.

Mix everything well in a bowl, if the tuna is in chunks - break it with a fork.
once the mix starts to look somewhat well-blended (we dont want the tuna to completely lose shape, just to be in tiny pieces) lay it out on a well-PAMed or oiled cookie sheet, I made just a rectangle shape... LAZY, can you tell??

Bake at 350-375 for about 15-20 minutes, I checked, when they were crispy underneath, I turned them and shut the oven but left the cookies in it, to finish cooking. They turn out semi-crispy (crispy on the edges, but chewy and easy to tear to pieces for training)
Here are the results:
1. Uncooked
2. Finished